When adults see an object occlude another object, they typically assume that the occluded object (a) continues to exist behind the occluding object; (b) retains the physical and spatial properties it possessed prior to occlusion; and (c) is subject to the same physical laws as prior to occlusion. Piaget (1954) claimed that infants do not begin to share these assumptions until about 8, 12, and 18 months of age, respectively. Over the past three years, we have conducted an extensive series of experiments designed to investigate Piaget's description of the development of infants' beliefs about occluded objects. The results we have obtained do not support Piaget's description. These results indicate that (a) infants as young as 3 months of age understand that objects continue to exist when occluded; (b) infants as young as 5.5 months of age are aware that objects retain their physical and spatial properties when occluded; and (c) infants as young as 9 months of age make inferences about occluded objects. In general, these findings indicate that young infants' understanding of the physical world is far more advanced than the work of Piaget and his successors had led us to believe. The first aim of the proposed research is to pursue a number of questions raised by our findings, namely: (1) Are infants less than 5.5 months of age able to represent the properties of occluded objects? (2) How sophisticated are 9-month-old infants at inferring the physical and spatial properties of occluded objects, and are younger infants also capable of making such inferences? Finally, (3) How can one account for the discrepancy between Piaget's conclusions and those suggested by our findings? The second aim of this application is to explore other facets of infants' physical world. Two physical domains will be investigated: (1) infants' understanding of support relations and (2) infants' understanding of collision events. Since our research has revealed that infants' understanding of object permanence is surprisingly sophisticated, it is reasonable to expect that investigations of other aspects of infants' physical world will also bring to light hitherto unsuspected competencies.